Frequency and speed control



March 25, 1930. F WUNSCH 1,751,539

FREQUENCY AND SPEED CONTROL Original Filed March 27, 1925 2 Sheets-Sheet2 Patented Mar..l 25, 1930 Unirse sra TES PATENT GFFECE FELIX WUNSCH, OFPHILADELPHIA, PENNSYLVANIA, ASSIGNOR TG LEEDS SE NORTH- RUP. COMPANY, FPHILADELPHIA, 'VANIA y Original application filed. March 27,

My invention relates to a system of controlling the frequency of auctuating or alternating current generator or the speed o a movingsystem.

ln accordance with my invention, to maintain the speed of a movingsystem substantially constant, variations in frequency of an lilalternator driven therebv produce "'eiiects upon a balanced electricalnetwork, as a lvVheatstone bridge, to control the position ci' meansadjustable to vary the speed ot said system; more specifically andpreferably, the sneed of the moving system is kent Within certain limitsby governor mechanism or equivalent, and the action of the governor isvaried in senses and to extents depending upon the magnitudes of theunbalancing effects produced by changes of frequency upon the balancednetwork.

My invention 'resides in a method and apparatus of the characterhereinafter described and claimed.

For an understanding of my method, and for an illustration of some ofthe various forms my apparatus may take, reference is to be nad to theaccompanying drawings, in which:

Fig. 1 is a diagrammatic view of apparatus suitable `for practicing mymethod and embodying my invent-ion in one of its forms; and Fig. la is amodification.

iq. 2 is a fragmentary perspective view of recording7 and controlmechanismV utilizable in accordance with my invention.

Fig. 3 is a diagrammatic view of a system for controlling speed orfrequency.

Figs. i and 5 are fragmentary views of modified forms of speed orfrequency-controlling means.

lteferring to Fig. l. A is a generator or source of alternating orlluetuatingcurrent Whose i'requencyis to be measured or indicated. ltsterminals are connected, respectively, to the conjugate terminalsorvpoints i and 2 of a llfheatstone bridge Whose other conjugate pointsare at the points of engagement of the contacts 3 and 4 movable alongthe slide Wires or adjustable resistances S and S', respectivcl ln thefirst bridge arm is included 'Y condenser C, shunted by thenon-inductive PENNSYLVANIA, a. conrona'rron or rmmsvnraEQnnNcY AND srnnncon'rnon 1925, Serial No. 418,7234. Divided and this application ledJune 27,

1928. Serial No. 288,688.

resistance r; in series with that portion r4 of the non-inductiveresistance S lying to the 'left ofthe Contact 3. In the second bridgearm are included the condenser C1, non-inductive resistance 75, and thenon-inductive resistance rs constituting the remainder of the resistanceS to the right of the Contact 3. ln the third bridge arm are includedthe noninductive resistance re and the portion a" oi the slide Wireresistance S1 to the right of the contact d. ln the fourth bridge armare the resistance TB constituting the remainder of the resistance S1 tothe left of the contact Li, and the non-inductive resistance r9.

ln case the resistances S and S1, which may be in the form of simpleWires or in the form of helices along which the contacts 3 and 4 move,are not by themselves of exactly correct values, they may, as Wellunderstood in the art of slide Wires, be shunted by resistances such asT10 and ru,respectively, to ensure between their terminals exactly thecorrect or desired amount of resistance.

Itv is desirable that all of the resistances aforementioned shall havesmall or substantially zero temperature co-elicient, to avoid influenceof temperature upon the measurements.

While the source A is in one conjugate conductor of the bridge, in theother, between the contacts 3 and 4, is connected the movable coil of lagalvanometer having the deflecting pointer or needle 19. |The coilWinding 20 may, as Well understood in the art, be shunted by aresistance T12, of suitable magnitude. The alternating magnetic tleld ofthe galvanometer is produced by current from the source A traversing thegalvanometer field Winding or coil f, in series with which may beconnected the resistance T13.

rlhe phase angle of the current exciting the galvanometer ield magnetwinding with respect to the impressed electro-motiveiiorce may change orvary from various causes, such, for example, as temperature changes ofthe winding f, change in magnitude of the impressed voltage, etc. Such achange of phase of the field-exciting current will introduce error inthe measurements unless 'I'ns are PTOVdsd L0 make possible a true`balance of the bridge regardless of the shifting of phase of thefield-exciting current of the galvanometer. To this end it is essentialthat three ratios shall be equal for all frequencies within the range tobe measured by the bridge. This equality of ratios is expressed by thefollowing equation:

T1 d1 a l -V in which: T1 and (Z1 are, respectively, the resistance andreactance components of one arm of the bridge, as, for example, theaforesaid -lirst arm including C, T and T4; T2 and d2 are, respectively,the resistance and reactance components ot another arm of the bridge,for example, the aforesaid second arm including T3, T5 and C1; and a andb are, respectively, the resistances or impedances of the other arms ofthe bridge, for example, the aforesaid third and fourth arms including,respectively, T6, T7 and T?, T9. y

In the example illustrated in Fig. 1, Where condensers are employed, itis desirable and preferable that they be loW loss condensers, to avoidphase angle or capacity changes With temperature changes, and arepreferably mica condensers of the besttype.

The combination of a condenser, as C, with a shunting resistance, as T,is the equivalent or has the characteristics of a condenser havingconsiderable losses, which may be expressed in terms oi an equivalentresistance which changes in magnitude With changes in frequency.Accordingly, the aforesaid resistance T1 is not the actual non-inductiveresistance T plus T4, but is an apparent, fictitious or equivalentalternating current resistance which, however, changes in4 magnitudewith changes in frequency, and comprises not only the resistances T, T4,but, in addition, the apparent or iictitious resistance corresponding7with the losses, it any, oi the condenser C per se, denser withaforesaid resistance T2 sum of the resist-ances T3 and T5 and theapparent or ictitious resistance, variable with frequency, due to thelosses of the condenser C1, if any. However, when the condenser C1 is ahigh grade mica condenser, with low loss, We may write its shuntingresistance T. rEhe corresponds with the T2=T-l-T5 (2) D lt is furthertrue that T farcrttt 3) and l ,wQL 4 1-P01202?2 and in Equations (3),(e) and (5), o (omega) and of the combination of the con-- is 2 ar timesthe irequency; C and C1 are, respectively, the capacities of thecondensers C and C1; and T and T4x are the magnitudes of the resistancesT, T4. Equations (3), (4) and (5) are .Well known to those skilled inthe alternating current art, and from them' are determinable the propervalues of the several resistances and capacities to be vutilized in anarrangement. such as indicated in Fig. 1 for measuringr a desired rangeot frequencies, it being understood that the contacts 3 and 4 are movedin unison with each other because attached to the same movable nicmber ccarrying the pointer d and co-acting with the stationary scale ecalibrated in terms or frequency, speed, or both, or any equivalent ofthem. rlhe contacts 3 and e take a different position for each differentfrequenin order to balance .the bridge for that frequency, in whichevent the galvanometer coil 20 is in its zero or non-deflectingposition.

From' the foregoing, it will be understood that the function of theslide Wire S1 is to change the ratio of the resistances in the third andfourth arms ot the bridge in proportion to the change in the ratio ofthe effective resistance and reactance in the first and second arms ofthe bridge in order that the equality oi ratios expressed in Equation(1) shall obtain, whereby the bridge may be truly balanced regardless otchanges oi the phase angle of the current in the galvanometer field f.

ldfhile measurement of frequency has been described in connection withFig. 1, it will be understood that speed or any other magnitudeproportional to frequency may similarly be measured. For example, thespeed of a rotating member may be measured by driving an alternatingcurrent generator, as A, by such member, and then measuring thefrequency of the current delivered by the generator.

it shall be understood that my invention includes equivalents of thearrangements shown in Fig. 1, for it Will be understood that in lieu ofor with resistances T6 and T9 may be utilized either inductive orcapacitative reactances, and that the slide Wire S1 may represent aninductive reactance, variable portions of which are included in theneighboring bridge arms, or may represent complementarily variablecapacities included in the neighboring bridge arms. Furthermore, it willb e understood that in the iirst and second bridge arms there may beused inductive reactances, and that the slide Wire S may be inductive,or complementarily variable capacities may be connected in theneighboring bridge arms.

in Fig. 1t1 is shown such a generally equivalent arrangement in Which inthe iirst bridge arm are the serially related inductance L andresistance T14, and in the second bridge arm the inductance L1 shuntedby a 'shaft 5 et substantially constant resistance rw.' t will beunderstood that the total resistance of the first erm of the bridgecomprises the resistance, the resistence of the conductor of theinductance L, end the resistance T14, which lest is present es e.sepsrete unit irn the resistance of the conductor or' of the inductenceL is not itself suiicient.

ln this case again lquetion (l) applies. r1 and all are the resistanceand reectance components of one erm oli the bridge, es for exemple, theiirst erm including L, fr, T14; r2 and d2 ere respectively theresistance end reectance components of another arm of the bridge, forexample the second erm including r3, r1, r1@ and L1.; and a and 25, arerespective ly, the resistances or impeden-ces of the other arms of thebridge. for example the third and fourth arms including respectively TB,7 eind Te, 7,9.

The arrangement of Fig. 1, or equivalent thereof, is utilizable forrecording,r and con-- trolling frequency and speed by recourse to anysuitable recording end control mechenism, one example of which isillustreted in Fig. 2, end corresponding 'with n structure of thecharacter disclosed in prior Letters Patent or the United Sitz-ttes toLeeds lio. 1,125,699, January 19, 1915.

Referring to Fig. 2, lv/l is en electric motor or other suitable sourceoil power the speed. i lever 6 is pivoted et its ripper end on ehorizontal exis and hes pivoted thereto on n. horizontal exis et itslovver end the erin l, on each end of which is e shoe 7a, of cork or thelilre, 'rictionelly -engeging the rim 8 of the clutch disc or vvheel 9,secured upon the shaft 1G of the movable structure or system. Secured.upon the shaft is e oem 11, which periodically engeges the lever 6 endmoves it outvverdly, in opposition to o. spring,` not shown, v therebyliiting the shoes 7a :from the rim 8, the spring returnin the shoes intoengagement with the rim after predetermined rotation oi the cern 1i.Upon the shaft 5 is secured e second cern l2, which,

after the shoes 7 have been lifted from the rim by the cern 11,dotnet-es the ringer 13 on the lovver end of the erin 1d secured et itsupper end to the member 15 pivoted on e horizontel exis et -16. Upon themember 15 is secured the member 1?, Whose upper edge 18 is inclinedoutwardly 'from the center,

,eee 3 Bisposed immediately above the edge 18 is the aforesaid needle orpointer 19 of the alternating current galvanometer whose coil is 20. Atthe opposite ends of the member 17 are the ebutments 2l for limiting thedeflection of the needle 19. Directly above the needle 19 end beneathwhich it normally freely swings are the edges 22, preferably straightand horizontal, upon the members 23, 23 pivoted et 2e, 24 and extendingtoward euch other, lleaving a gap of sucient Width between their innerends for free entry of the needle 19 when in balanced or zero position,corresponding with e balance of the lfVheatstone bridge of the charactershown in Figs. 1 and 3. The needle 19 normally swings freely between theedge 18 and the members 23, 23, which letter have downwardly extendingarms 25, 25, biased toward each l other by the spring' 26. Attached tothe loW er end of the erm or lever -6 is o. triangular plate 2'(carrying the pins 28, 28, fio-operating with the lower ends of themembers 25, 25. it opposite ends of the erm 7 ere the lugs 29, 29,adapted to be engaged by the cams 30, 30,'similzir in shape andsimilarly positioned and secured upon the shaft 5.

As the gelvanometer needle 19 deflects in the one direction or theother, the shaft will he rotated inthe one direction or the other, andto en extent corresponding to the extent or the gelvenometer deiiection.For exemple, when the needle 19 deilects to the rig-ht, Fig. 2, due toperiodic vertical movement of member by cern 12, the needle 19 isclamped between the inclined edge 18 and lov/er edge 22 of the righthand member 23 causing the erm 25 to be tilted in e clockwise directionabout its pivot 24, thereby pushing the right hond pin 28 on plate 2?,end so tilting; the movable or driving clutch or erm 6 in n cloclrvvisedirection Tvhile cem 11 is holdingl shoes 7, i7 from the rim 8 of clutchWheel 9, the angular movement oit the niemer 8 being dependent upon thedecree or lferlection oi needle 19. The cern llsoon thereafter ellovsthe shoes to again grip the rirn 9 off Wheel 9 end soon thereafter theleft hand com 3G engages the eer 29, which has been elevated, pushing itdownwardly due to rotation oit shaft 5, to restore it to the horizontalposition indicated, but in so moving beclr normal position the clutchmember rotates the disc 9 sheft 19 eilixed thereto in ocounter-clockwise direction. i occured upon the sha-F510 is e disc 51 ofinsulating; materiel, carrying upon its "periphery the eoreseidresistences or impedences S, LS1, which in this exemple, as indiceted inFig. 3, ere rotated, While their conctinr contacts Send el erestationery. Secure' upon the shaft 1 0 is e second disc 32 carrying' thearcuate contacts 33 and 34:, with which co-ects the stationery Contactor brush The discs 31 and 32 ere ndjusteble to eny suitable angles withrespect to each other and with respect to the shaft by suitable` set4 40and has attached thereto the pen or marker 41 for drawing upon therecord sheet or paper 42 a mark or record 43, of the chan es of wfrequency or speed. The paper 42 is driven by the motor M, aswellunderstood in the art. Disc 31 may carry the frequency or speied scale eco-acting w1th stationary point er g5 Referring to Figli, the movablestructure or system of Fig. 2 is shown in so far as it comprises theshaft l0 and the discs 31 and 32. The slide wires S, S1 are comprehendedin a Wheatstonebridge 'of the character indicated in Fig. 1. i l

lli-turbine or other source ofpower, or generically a rotating system,T, drives the alternating current generator or source A related to thebridge of Fig. 3 as thesource'A as is related to the bridge in Fig. 1.The source A in this instance may be a small alternating currentgenerator driven by the rotary structure T through the shaft 44 forpurposes merely of measuring or controlling the speedotthe to device T.0r it may be an alternating current generator of large capacity forsupplying current to any suitable load connected to the suppl circuitconductors 45 and 46, in which case t e fre uency or speed oi T and A.may B be controlle by controlling the opening of the valve V controllingthe rate oi supply ci motive fluid or steam throu h the pipe 47 to theturbine T. Tn this case t e brush with contacts 33 and 34 control thereversible eleca@ tr'icmotor M1 receiving current from any suitablesource, as 48. When the disc 32 is in the position indicated in llig. 3,the motor M1 is e-ener ized, and this condition corresponds with ba anceof the eatstone bridge for.

the desired speed or trequenc of the turbine T or generator A. Should te speed or irequency rise, the Wheatstone bridge will be unbalanced,causing a dedection of the galvanometer and its needle 19, causing inturn eo a rotation of the shaft 10 to an extent corresponding with theextent o'unbalance of .the bridge, and in such direction that thecontact 33, for example, Will be brought into engagement withthe'contact 35, energizing the motor M1 which runs in a certaindirection,

rotating the shaft 49, through any suitable lgear reduction, genericallyrepresented by the pulleys 50, 51 and belt 52, thereby causing themember 53 to advance upwardly along the to threaded shaft 49, carryingthe abutment 54 with' resultant tendency to compress the spring 55,lwhich opposes the y ball governor 56, driven b the turbine or device T,to partially close t e valve V, caus- G ing reduction in speed andfrequency. Simiupwardly,

menace larly, when the frequency or speed diminishes, the motor M1 willbe energized through contacts. 34 and 35, and will rotate in t eopposite direction, and eiiect an increase in the opening of the valveV, with resultant increased speed and frequency. In both cases the speedor frequency is varied or changed toward the desired frequency,

at which the contact 35 is 1n engagement with neither of the contacts 33and 34, and the bridge isV in balance at the desired frequency. When thefrequency or speed changes from the desired magnitude, the l/Vheatstonebridge is unbalanced, the galvanometer detlects, and the discs 31 and 32are rotated by the shaft' 10 in a direction tending to rebalance thebridge at the impressed frequency and cause a change in the speed orfrequency back toward ation of speed or frequency is materially reducedto a practically negligible amount. The speed or frequency may beconsidered substantially constant, less than 116 ci 1% variation beingobtained in actual practice. At the same time, the automatic systemproduces a record of the speed orirequency upon the record paper 42, orindicates speed or frequency by scale e -on disc 31.

ln lieu of the valve control mechanism indicated in Fig. 3, there may beutilized that indicated in Fig. 4, where through suitable reducinggearing, the motor M1 rotates the valve stem to eiiect partial closureor opening of the valve V upon increase ory decrease Y of speed orfrequency. lnv either of the valve arrangements of Figs. 3 and 4, it`will be understood that the valve may be either in the main steam ormotive fluid line, or may be in a by-pass, and that the governor 56 ofFig. 3 may be either the main governor or an auxiliary governor oftheturbine T.

ln Fig. 5 there is fragmentarily indicated an alternating ycurrentgenerator or source A, either solelyztor speed measurement, or forsupplying current to a consumption circuit, asin Fig. 3. Tn this case,however, the turbine T is replaced by an electric motor M2, such, forexam le, as a shunt motor, hav.- ing the armature 5 and shunt field 58,with a variable resistance 59 in the armature circuit, or in the fieldcircuit ii desired, and adjusted by the contact 60 moved b thenut 61movable along the threaded sha 62 driven loo las

reversible motor M1 ofFig. 8.

w .bisher frequencies,

20 aient magnitude ebrougli suitable reduction gearing by the ln ibaoperation of a frequency or speed contini si stem of thecharactervliereinbeoie 5 describe the frequency of an alternaiin oruciuaiing current is in e'ect determine or measuied, as in ille case ofFigs. l and l, or equivalent, and in addition, a frequency or speedcontrol is eiected.

Iii will be further understood ibut my inveniion is not limited tocontrol 02E frequencies of 25, 60 or more cycles per second as commonlyutilized on power and lighting circuits, but comprehends also controloi' including the so-called nigh oi' iadio frequencies.

non breviiy in the ap ended cleims, tbe terni $6frequency is emp oyed ina generic sense io include frequency, s eed, or equivproportiona lo .ordependent upon icquency or speed. I

This application is a division o' ini copending application Serial No.16,7%, led Merch e7, i925.

1ylll/'bei l claim is:

l. ln e system comprising a Wheaisione bridge having combinations ofresistance and veaciance in di'eient bridge arms end moveble stiuciureassociated with said bridge, -alle method of controlling frequency,*which commises impiessing a ucbuuting cuen-.ni representative of thefrequency 'io be con iio'lled upon said Wheatsione bnidge, eiieciingmovement of said structure in iesponse i ioand io en oscene dependenilupon iiie de= grec ci? unbalance of elle bridge, eiifecfcing by seidmovement variation ori the reiuiive mog niiudes of impedances in chebridge och Wand equality becvveen tbe initio oi' olie resisiencecomponents ci svvo arms oi bridge, ibs seein o' 'the ceaciancecomponents of ci ii-:lie biidge, and inizio oi isbe impede-.noos offiabe other alfine osi tbe bridge, ''siie :frequency of seid eussent bysaid nopeiiaius 'for coniiiolling heenweg, compiising a lViieetstonebridge bovine; combinalons oi2 reaciance and resistance in dii'i'e'entbiidge omis, impedances in Jone ciber biidge enne, a souce of duciuaingconvene.

in a conjugate conductos of said biidge, a uciueiing curi'entgalvenoineber in Lelle einen conjugate coifiducifoi7 oi said bridge,ineens foi exciting the golvanomeier :eld

said source, movable structure con- 'iiolied by said galvenonietei when`seid beide-e is unbalanced, and incansconlisrolled by said movablestiucture for adjusting tbe veleiive magnitudes of impedance in 'thediiifei'ent peine of biidge aiins iovvuid equality bobiveen'ibe ratio cithe resistance componente of said rst named ams of said bridge, ellesabio off the seacbancecomponents of said iiifsenenied arms ci saidbridge, and tbe ratio ci one impedances o said other arms oi lsaidbridge, and means *for varying the frequency of `the current from saidsource controlled by said movable structure. e

3. ln a system comprising e. moving system, an alternator driventhereby, an electrical balancing neiwoilnand means controllingAeiiei'gizeiion of said moving system, the method ci controlling thespeed of said inovinrj system which comprises, pioducing unbe ance ofsaid electrical ncivvo'flz by departuie of bhe frequenc of bhealternator i'oni a predetermined iiequency at which tbe network isbalanced, and utilising the unbalanced network to modify the action ofsaid means to control delivery of motive eneigy to seid system in asense and to an enten?. more or less completely to compensate for saiddeparture.

e. ln a system comprising a moving system, means for regulating deliveryof energy thereto to maintein the speed within a range, an alternaitoidriven by said sysein, and an elecrical balancing network, the method cicontrolling the speed of' the moving system, which comprises producingunbalence of said electrical netvvcilz by depaituie oi" tbe ireoguenc oiseid elienaiionfiono e credete?n mine frequency ai: which ille neivloliis belanced, and. utilising the unbalanced network to .modify 'elleaction of said negnlaiing ineens.,

5. In a system compiising an nieinaco, an elecifical baiuncing network,and moveble snuctuie essociaied Willi seid wie ineicbod of conisnoilingtbe f ecquency of "oleolieineioi v/bicii conipi'ises i C iucuaiinecuireni epnesente'ive ci soif piedeierinined frequency, eiifec'ii'ngneoveinent of said siiuciu'fe in nesponse lo end io enen'i depinidenilupon ibo degree o; unbolence of tbe newoil, and by said inovezfnenlveriaiion in 'elle magnitude of inoiive eneigy epplied io saidel'eineloi:

6. in a sysem ccnipnising an slernntoi, eleci'eical balancinff network,ond movable structure associated with seid network, .ille inetbcdcontiolling the :frequency of +elle alieinaioi' which compiisesimpressing e fluctuating curieus iepiesentaiive of said ire guency uponsaid neivvoik belanced ai a 'pieeiecmined cequency, efiecting .movementof seid siiuciuie to iiebalonce said network upon and in response todeparture oi' die frequency ieoin said oedezerinined iieouency, andproducing bi said .novenueniI valuation in the of incisive energyoppliedio said velieivieion a sense and to an eisien'f, 'to com-"ponente ion seid depaiiuie.

7. ln a system 'comprising en alteineior, a Wiieaibsione bridge havingcombinations of resisance and neactance in die'feni, bridge siens,movable siructuie associated said bfidge, and mechanism relaied 'io seidstruciuie coniiolling motive energy a ,olied lo said ni'ecncio, 'llocmethod ci contvolling tbe ireiso vquency of the4 alternator whichcomprises imand controlling by said movement the position of saidmechanism regulating the'magnitude of motive energy applied to saidalter-l nator to adect the speed thereof: i

8. A system of control comprising an alternator whose frequency is to becontrolled, motive means driving said alternator, anvel'ectricalbalancing network upon which is inipressed |a uctuatingelectro-motive-force representative of the frequency of said alternator,said network balancedat a predetermined frequency, movable structureassociated with said network actuated I balance of the same in responseto a diderent frequency of said electro-motive-orce. and meanscontrolled by .said structure for varying the speed oit said motivemeans to correct the frequency oi" said alternator.

9. A. system of control comprising an alternatorwhose frequency is to'becontrolled, motive means drivin said alternator, governing means :tormaintaining the speed of said motive means within a ranGe ofpredetermined limits, an electrical ba ancing netquency of saidalternator,

work upon which impressed a ductuatning electro-motive forcerepresentative of the rreanced at a predetermined frequency, movablestructure associated with said network actuated upon imbalance of thesame in response to a different 3frequency of said electromotive-force,and means controlled by said structure coacting with said governingmeans to modify its action whereby the frequency oi' said alternator ismaintained substantially constant. i0. er system comprising analternator, a prime mover. tberefonaa governor maintaining tbe speed ofsaid prime mover substantially constant, and means for determining thegovernor setting comprising an electrical network responsive to thealternator frequency and means operable when the network is not balancedat a predetermined frequency to change tbe setting of said governor.

11. Apparatus for controlling frequency comprising a Wbeatstone bridgehaving combinations. of reactance and resistance in different bridgearms and impedanoes in other `bridge arms, an alternator in oneconjugate conductor or" said bridge, electro-responsive means in theotber con] agate conductor, strueture controlled by saidelectro-responsive means when said bridge is unbalanced foradjusting-the relative magnitudes of impedance in the' diierent pairs'of bridge arms toward balance of` said bridge at alternator frequencyZand means eective 4when said structure is displaced from i.predetermined upon unsaid network balposition to vary the alternatorspeed in a sense decreasing the diderence between tbe actual alternatorfrequency, and the desired frequency. s

12. Apparatus for controlling frequency comprising a Wheatstone bridge'responsive to the frequencyof an alternator in circuit therewith,dedecting means responsive to unbalance of said bridge, structure movedby said deecting` means to an extent and in a sense from a predeterminedposition dependent upon the imbalance and means effective when saidstructure is displaced from said predetermined position for varying thespeed of the alternator to restore balance.

13. An arrangement comprising an alternator, a prime mover therefor, apower supply circuit connected to said alternator, a governor formaintaining the speed of said prime mover substantially constant, asystem balanced at a predetermined frequency including a Wheatstonebridge connected to said alternator, and means associated with saidsystem actuated in accordance with unbalance thereof to change thegovernor setting in a sense and to an extent dependent upon the senseand extent of unbalance of said system.

FELX W'UNSH.

